This invention relates to shutter mechanisms for graphic apparatus. It relates, more particularly, to shutters whose primary function is the control of the transverse section of a light beam, rather than the period of illumination. Existing shutters are quite adequate to serve this function in most types of conventional camera and projection equipment. However, they are generally not satisfactory for use in photographic pattern generating machines, such as those typified by the pattern generator described in U.S. Pat. No. 3,722,996, commonly utilized to produce tooling plates for electronic microcircuits and solid state devices.
The primary requirement for such a shutter mechanism is the precise definition of a bounded orifice corresponding in shape, proportion and orientation to the desired pattern or pattern element on the finished product. The precision necessary to achieve the desired performance in the product is typically meausred in micro-inches. Therefore, even if the pattern generator is employed to produce a transparency at ten times the ultimate dimension of the tooling place, to be photographically reduced in a later stage of the production process, the aperture of the shutter has to be controlled within very narrow tolerances.
It is a further object of the invention to combine such precision with ease of operation in a relatively uncomplicated, economically produced mechanism of durable construction which requires little service or maintenance.
The invention attains its objects by providing a pair of planar shutter blades, reciprocable relative to each other, whose confronting edges define the aperture of the shutter. The size of the aperture is controlled by moving both shutter blades simultaneously to vary the gap between them. The shape of the aperture is defined by the configuration of the confronting edges. A mechanism employing shutter blades with parallel straight edges will produce a slit of variable width as the light-passing orifice.
The reciprocal motion of the shutter blades is achieved by providing an endless belt loop, most suitably formed by a flexible steel band, in the plane of the blades. A pair of spaced capstans or a combination of capstans and idler pulleys support the endless belt in a peripheral pattern having two parallel regions equally spaced on either side of the intended path of motion of the centers of the shutter blades. One blade is anchored to one side of the loop and the other blade is attached to the other side of the loop. A suitable power source connected to the capstan system drives the endless belt and causes the attached shutter blades to move in opposite directions.
The drive system is reversible, to allow the shutter blades to be moved to any desired positions equidistant from a predetermined aperture center, within the area bounded by the parallel portions of the belt loop.
In most instances the light transmitting aperture must be defined by more than two movable edges. Two or more shutter blade pairs, each provided with its endless belt suspension and drive, may be superimposed on each other with their axes of travel displaced at an angle.
As will be seen, the endless belt drive of the subject invention has many advantages over prior art devices relying on racks and pinions or pushrods for shutter blade movement. Its response is direct and instantaneous. Its movement is precisely controllable and repeatable. It avoids the need for camming surfaces in the drive mechanism, thereby eliminating play and backlash and it is substantially unaffected by friction and wear. These and other advantages will become clear from the following detailed description of several preferred embodiments, given with reference to the accompanying drawings.